An enzyme which is capable of N-acetylating serotonin or tryptamine was routinely purified about 350-fold from rat liver high speed supernatants. The apparent Km for the substrates acetyl CoA and tryptamine were found to be similar to that of an enzyme performing a similar function in the rat pineal gland. The apparent molecular weights of the liver and pineal enzymes were also found to be essentially the same. The pH optimum for the liver enzyme was in the vicinity of pH 8.5 but the optimum of the pineal enzyme was pH 6.5.